Cytochrome c models have been synthesized to probe three structural and thermodynamic features: the iron-ligand bond length changes upon redox, the intrinsic ligation ability of thioethers to hemes and the contribution of methionine ligation to the high redox potential of the cytochromes c. The Xray crystal structures of bis (tetrahydrothiophene)-(meso-tetraphenylporphyrinato)iron(II) and bis(pentamethylenesulfoxide) (meso-tetraphenylporphyrinato)iron(III) perchlorate reveal Fe-S bond lengths (2.32 - 2.35 A) which are quite insensitive to oxidation state change. That the latter complex is six-coordinate and low-spin suggests that a thioether is a better ligand to Fe(III) than previously believed. A 'tail porphyrin' with an N-imidazole ligand covalently attached via a valeryl amide linkage to meso-(o-aminophenyl)triphenylporphyrin has been synthesized in order to prepare a mixed ligand imidazole-thioether ferrous porphyrin complex. The Xray crystal structure of this cytochrome c analogue reveals a short Fe-NIm bond. Electrochemical studies on these complexes suggest that the thioether-imidazole heme ligation mode is responsible for about 160mV positive shift in the Fe(II/III) redox potential relative to bis-imidazole ligation. This coming year we will attempt to prepare a Fe(III) cytochrome c analogue. Also we will begin new work on a cytochrome oxidase model whereby a copper complex will be covalently linked to an iron porphyrin. The object of this synthesis will be to prepare ligand bridged Fe-X-Cu systems.